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I'm in a beginner's java class. This Lab is for me to make a class "Wallet" that manipulates an array that represents a Wallet. Wallet contains the "contents[]" array to store integers represing paper currency. The variable "count" holds the number of banknotes in a wallet. After writing methods (that match provided method calls in a serpate Driver class) to initialize the Wallet and add currency/update "count", I need to transfer the array of one instantiated Wallet to another. I don't know how that would work because the one Wallet class has only been messing with a wallet called "myWallet" and now I need to take a new Wallet called "yourWallet" and fill it with "myWallet"'s array values. //I should note that using the Java API library is not allowed in for this course

My Wallet class looks like this so far:

public class Wallet
{
    // max possible # of banknotes in a wallet
    private static final int MAX = 10;

    private int contents[];
    private int count;      // count # of banknotes stored in contents[]

    public Wallet()
    {
        contents = new int[MAX];
        count = 0;
    }

    /** Adds a banknote to the end of a wallet. */
    public void addBanknote(int banknoteType)
    {
        contents[count] = banknoteType;
        count = count + 1;
    }

    /**
     * Transfers the contents of one wallet to the end of another.  Empties the     donor wallet.
     */
    public void transfer(Wallet donor)
    {
        //my code belongs here
    }

...

The Driver code looks like this:

public class Driver
{
    public static void main(String args[])
    {
        Wallet myWallet = new Wallet();

        myWallet.addBanknote(5);
        myWallet.addBanknote(50);
        myWallet.addBanknote(10);
        myWallet.addBanknote(5);

        System.out.println("myWallet contains: " + myWallet.toString());

        // transfer all the banknotes from myWallet to yourWallet
        Wallet yourWallet = new Wallet();
        yourWallet.addBanknote(1);
        yourWallet.transfer(myWallet);
        System.out.println("\nnow myWallet contains: "
                                                 + myWallet.toString());
        System.out.println("yourWallet contains: "
                                               + yourWallet.toString());

I want to use addBanknote() to help with this, but I don't know how to tell the transfer() method to transfer all of myWallet into yourWallet.

I had the idea to do somethign like this in transfer(): 

yourWallet.addBanknote(myWallet.contents[i]);

with a traversal to increase i for myWallet contents. It seems horribly wrong, but I'm at a complete loss as to write this method. If my problem is so unclear that nobody can help, I would be more than happy to receive advice on how to ask a better question or on how to search with correct terms. Thanks for any help you can provide.

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4 Answers 4

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I don't want to spoil your homework as you seem to be going the right way, but I do have some comments which you may either take or not :)

First, I would probably put the bank note types in some enumeration. But as that sounds a bit to advanced, consider

public class Wallet {
    public static final int ONE_DOLLAR_BILL = 1;
    public static final int FIVE_DOLLAR_BILL = 5;
    ...

    // looks a bit more readable to me
    myWallet.addBanknote(ONE_DOLLAR_BILL);

Transferring all the banknotes from the donor to yourself should not be so much of a problem (a for loop would do) but I think you're in a world of hurt if you are trying to implement a 

removeBanknote(int banknoteType);

as you are using count not only as a length but also as an index variable. By this I mean that you assume contents[0] ... contents[count-1] hold valid banknotes. And how do you remove one without too much work?

Warning: a bit more advanced

In your case I would probably opt to have a banknoteType of 0 indicating an empty banknote slot in your wallet, and implement _addBanknote(int banknoteType) as:

public void addBanknote(int banknoteType) {
    for (int i=0; i < contents.length; i++) {
        if (contents[i] == 0) { 
           contents[i] = banknoteType;
           count++;
           return; // OK inserted banknote at the first empty slot
        }
    }
    throw new RuntimeException("Wallet is full");
}

This may be a bit overwhelming at this point. But it would allow you to implement:

public void removeBanknote(int banknoteType) {
   for (int i=0; i < contents.length; i++) {
       if (contents[i] == banknoteType) {
           contents[i] = 0; // better: NO_BANKNOTE = 0
           count--;
           return;
       }
   }
   throw new RuntimeException("This wallet does not contain a banknote of type " + banknoteType);
}

Please note that in both methods I return when I successfully removed or added the banknote. Only when I could not find a free slot, or the requested banknote, I finish the for loop and end up throwing an exception and thereby stopping the program.

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Comments

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I think the question is fine and I think you're on the right path. The way you're calling Wallet#addBanknote(int) is correct. What you have said is the right thing:

public void transfer(Wallet donor)
{
    // Traverse the donor's wallet
        // Add the bank note from the donor to this wallet
        // What do you think also needs to happen to make sure
        // the donor is actually giving their bank note?
}

Just another thing, what would happen in your Wallet#addBanknote(int) method if you have more contents than the MAX?

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4 Comments

I think transfer(Wallet donor) means 'transfer all notes from the donor to me'. tranfer(Waller donor, int bankNoteType) would mean 'transfer 1 single note of the given type from the donor to me'. And personally I would have chosen transferTo(Waller receiver) as giving money feels more natural in an api than taking it :)
@extraneon I think those are good questions and concerns for the OP. I was under the assumption (perhaps incorrectly) that they were given some sort of template and had to fill it in. I don't see your point about bankNoteType, as bank notes are just the value of the bank note (unless I'm mistaken). So the contents would be like: [1, 5, 20, 20, 100].
@mangoDrunk What does the '#' do in the last line you wrote? You are correct about both the template Wallet with missing methods and that the arrays in question are just int values. Also, extraneon Thanks you for explaining the method call! It was(and still is) one of of most unclear parts of my problem. What is that called? Method Parameters or is there a better name?
@DiscipleApollos Sorry, replace # with . wherever you see #. It's just a way to document things (it's not proper syntax). Method parameters is right.
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You can create either a constructor that takes another wallet, or a function (as already mentioned) and use System.arraycopy to copy the array in one fell swoop. System.arraycopy is fast, and its definitely overkill for something small like this, but its good tool to have in your toolkit.

The other alternative mentioned, copy the elements from one array to the other element by element in a loop will work fine too.

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In this case, I don't think I'm allowed to use pre-made methods. I'm writing down System.arraycopy for later tho. Thanks for the help!
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The myWallet inside the transfer method is named 'donor', and with that, it doesn't look horribly wrong:

addBanknote (donor.contents [i]);

You just need a loop around it, and to remove the yourWallet. which is the name of an instance of that class. That instance is inside the Class/method this, but needn't be specified, because there is no other addBanknote-Method in scope, which could be meant. (Thanks to mangoDrunk).

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1 Comment

yourWallet should also either be this or removed.

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